The Sun deposits 120,000 terawatts (TW) of power onto the Earth's surface. This is more than the 13 TW of total power that is currently used by the planet's population. Photovoltaics (PV) convert solar energy into direct current electricity using semiconducting materials that exhibit the photovoltaic effect. The photovoltaic effect includes photons of light exciting electrons into a higher state of energy, allowing them to act as charge carriers for an electric current. A photovoltaic system can employ solar panels including a number of solar cells to supply usable solar power.
Organometal trihalide perovskites (OTPs, e.g., CH3NH3PbX3, X=Cl, Br, I or a mixed halide) can be excellent low-cost, earth-abundant photovoltaic materials due to proper bandgap, excellent crystallinity, and strong absorption. In addition, OTPs have advantageous optoelectronic properties, such as a very large carrier mobility comparable to silicon, bipolar transport, and large charge carrier diffusion length, which enables high performance devices with the traditional planar heterojunction (PHJ) structure.
Perovskite photovoltaic devices (PPVs) have been demonstrated to have all the desired properties of organic photo voltaic devices (OPVs), which have strong market potential in military and civilian applications, including for flexible, wearable, lightweight, and portable chargers for electronics, building-integrated photovoltaics (BIPVs), and off-grid power generation. PPVs have shown all three main competencies of that OPVs have over other photovoltaic technologies: 1) PPVs can be made on flexible plastic substrates; 2) PPVs can be fabricated with low cost materials and a solution process; 3) Perovskite materials have tunable color and bandgaps with semitransparency, which allows for the integration of PPVs into buildings. State-of-the-art PPV devices can have an efficiency of 20%, but they need to reach 25% to compete with other commercialized thin film solar cell technologies in order to make them commercially viable. The thermodynamic efficiency limit of single junction PPVs can be 38% based on its bandgap.
High mobility and lifetime is important for photonic devices because the light-generated electrons and holes can move longer distances to be extracted as current, thus avoiding release of their energy as heat by the recombination. High mobility and high carrier lifetimes are also crucial for photo detectors. For example, in an organic/inorganic hybrid photo detector, if one type of carriers (e.g., electrons) can be trapped longer, the other type of carriers (e.g., holes) thus circulates many times with high mobility through the polymer matrix or network. In this case, ultra-high gain (gain may be defined by the ratio of the lifetime of the trapped electrons and the transit time of holes), can be obtained.
Despite the high efficiency reported in devices fabricated by thermal evaporation, the complicated controlling of the non-stoichiometry of OTPs such as CH3NH3PbI3-xClx by co-evaporation under high vacuum dims its advantage of being low cost. Low temperature solution processes are attractive in the fabrication of electronic devices, especially large-area solar cells, for reducing fabrication costs.
Thin film photovoltaic devices based on OTPs are a promising area of renewable energy research. These solar cells utilize light harvesting thin films include a polycrystalline perovskite with a structure of AMX3, where A is an organic and/or inorganic cation, (e.g., methylammonium (CH3NH3+(MA+), NH2CH=NH2+, (FA+) Cs+)), the M is a metal cation, (e.g., Pb2+, Sn2+), and X is a halide anion (e.g., I—, Cl—, and/or Br—). Power conversion efficiencies (PCE) of perovskite solar cells using these methods can exceed 16%. The solution processability of OTPs enables the capitalization of development in organic photovoltaics (OPVs) for low-cost, high-throughput production of OPT solar panels on both rigid and flexible substrates. However, current methods in fabricating high efficiency perovskite devices (e.g., solar cells), such as spin-coating or thermal evaporation, are not compatible with large-scale production processes, such as a roll-to-roll process. There is lack of a film deposition methodology for perovskite films which can be compatible with roll-to-roll fabrication processes for large scale production of perovskite solar cell panels while still achieving high performance.
Accordingly, there is a need for improved photoactive devices including OTPs as an active layer as well as improved methods for fabricating such photoactive devices.